In “Experiments in Aerodynamics,” I have given the result of trials, showing that the pressure (or total resistance) of a wind on a surface 1 foot square, moving normally at the velocity of 1 foot per second, is 0.00166 pounds, and that this pressure increases directly as the surface of the plane, and (within our experimental condition) as the square of the velocity,[20] results in general accordance with those of earlier observers. I have further shown by independent investigations that while the shape of the plane is of secondary importance if its movement be normal, the shape and “aspect” greatly affect the resultant pressure when the plane is inclined at a small angle, and propelled by such a force that its flight is horizontal, that is, under the actual conditions of soaring flight. I have given on page 60 of “Aerodynamics,” the primary equations, ‹P›_α = ‹P›_{90}‹F›(α) = ‹k›‹A›‹V›^2‹F›(α), ‹W› = ‹P›_α cos α = ‹k›‹A›‹V›^2‹F›(α) cos α, ‹R› = ‹P›_α sin α = ‹k›‹A››V›^2‹F›(α) sin α, where ‹W› is the weight of the plane under examination (sometimes called the “lift”); ‹R› the horizontal component of pressure (sometimes called the “drift”); ‹k› is the constant already given; ‹A› the area in square feet; ‹V› the velocity in feet per second; ‹F› a function of α (to be determined by experiment); α the angle which, under these conditions, gives horizontal flight. I have also given on page 66 of the same work the following table showing the actual values obtained by experiment on a plane, 30×4.8 inches (= 1 sq. ft.), weighing 500 grammes (1.1 pounds): Weight with planes Angle Soaring speed Horizontal Work expended of like form that with ‹V›. pressure per minute 1 horse-power will horizon ‹R›. 60 ‹RV›. drive through the α. air at velocity ‹V›. ------- ------------- ---------- ----------------- -------------------- Metres Feet Grammes. Kilogram- Foot- Kilo- Pounds. per per metres. pounds. grammes. sec. sec. ======= ======= ===== ========== ========= ======= ========  ======= 45° 11.2 36.7 500 336 2,434 6.8 15 30 10.6 34.8 275 175 1,268 13.0 29 15 11.2 36.7 128 86 623 26.5 58 10 12.4 40.7 88 65 474 34.8 77 5 15.2 49.8 45 41 297 55.5 122 2 20.0 65.6 20 24 174 95.0 209 It cannot be too clearly kept in mind that these values refer to ‹horizontal› flight, and that for this the weight, the work, the area, the angle and the velocity are inseparably connected by the formulæ already given.